This invention relates to an apparatus for closing the discharge aperture of a metallurgical vessel. More particularly, this invention relates to an apparatus for closing a tilting converter by means of a closure element that is attached to a pivoted lever in order to prevent undesirable discharge of slag at the beginning and end of the casting process and during refining.
It is well known in the art that when a converter is rotated at the beginning of its evacuation process, lower density slag is the first material to enter the discharge aperture before the steel bath has a chance to fully cover the aperture and then flow therethrough. This situation has adverse effects on the service life of the fireproof lining of the pouring ladle since highly oxidized slags are extremely destructive. Moreover, when slag flows out of a carburizing furnace together with or prior to the molten steel, the slag is reduced by the alloying additives. This tends to decrease the yield of alloying additives, draws undesirable phosphorous from the slag into the molten steel and increases the occlusion content phosphorus in the steel.
At the end of the casting cycle and before the converter can be swung back into its original position, some additional slag will again flow with the steel into the ladle. The problem has resulted in the development of various devices including the ceramic ball or plug float. These ceramic balls or plugs have a density between that of the metal and slag and function to obstruct the discharge aperture to prevent slag flow after most of the steel has flowed out. However, since the shape of the discharge aperture is constantly changed by the abrasive and destructive environment of the molten metal, the heavy float method is not particularly reliable for preventing slag/steel mixing.
One solution to the above-described problems is suggested by German Offenlegungschrift No. 2,639,712 which discloses a device wherein a closure element, insertable into the discharge chamber, is provided for closing the discharge aperture of a metallurgical vessel by means of a compressed fitting and compressed gas flow which, during operation, pushes the slag back into the vessel as it is about to be drawn through the aperture with the steel. Argon or nitrogen are examples of compressed gases that may be employed. Because of the hostile environment, this device is subjected to considerable abrasion, especially in large converters.
An improvement to the closing device disclosed in the German patent discussed above has been to construct the outer jacket of the closure element in the shape of a calotte shell that merges on its opening side into a frustrum-shape surface. This improved device is shown in European Patent Application No. 10 082. In a preferred embodiment, the closure element is constructed of cast iron.
It is clear that both of these prior art devices suffer from the disadvantage that large quantities of gas are required in order to seal the discharge aperture. This excess of gas can create numerous problems including premature cooling of the molten steel. Moreover, gas flow must be continuously carried out in order to avoid loading in the mouth of closure element by the molten bath.